1. Field of the Invention
The present invention relates to an ink jet recording apparatus, and an ink jet recording method for recording information such as characters, images, and pictures on a recording medium. Also, the present invention relates to an information-processing system such as a copying machine, a facsimile machine, a printer, a word processor, and a personal computer, using such apparatus as its output device.
2. Description of the Related Art
Heretofore, various kinds of recording apparatuses for performing an image formation on a recording medium, for example, a sheet of recording paper, a piece of fabric, and a sheet of plastic material such as one used for an overhead projector (generally called as a OHP sheet in Japan) have been proposed as in the form of mounting a recording head in the type of wire dot matrix recording, thermal recording, thermal transfer recording, ink jet recording, or the like.
Among the conventional recording methods, the ink jet method has been known as one of the non-impact methods and mainly grouped into two different types from the points of how to prepare ink droplets and how to generate energies for ejecting them. That is, one is a continuous type, further grouped into a charge particle control type and a spray type; and another is on-demand type, further grouped into a piezo type, a spark type, and a bubble jet type.
In the case of the continuous type, a plurality of ink droplets are ejected continuously but only a required part of them is charged so as to be adhered on the recording medium while the others are of no use.
In the case of the on-demand type, on the other hand, ink droplets are ejected when they are required for the printing. Therefore, the ink can be used without causing any useless droplets and thus the internal parts of the ink jet apparatus are not stained. According to the on-demand type, furthermore, a response frequency of the recording means is lower than that of the continuous one because the on-demand type is able to start and stop the ink ejection. Therefore, the on-demand type realized a high speed printing by increasing the number of the orifices to be formed on the recording head.
Accordingly, many ink jet recording apparatuses being commercially available are provided as the on-demand type and thus they have been used as output devices of data-processing systems because of their high-density and high-speed recording behaviors. For example, the ink jet recording apparatuses have been provided as printers as output terminals of copying machines, facsimile machines, printing machines, word processors, work stations, or the like, or provided as handy- or portable-printers, personal computers, host computers, optical disc- or video-equipment, or the like. In general, furthermore, each ink jet recording apparatus can be constructed so as to meet the demand (for mechanical apply, business or private use, or the like) of each system described above, respectively.
The ink jet recording apparatus generally comprises: a carriage for carrying a recording means (i.e., a recording head) and an ink tank; a transfer means for transferring a recording medium (e.g., a sheet of recording paper); and a control means for controlling the drive of these means. During the recording period, the ink jet recording head moves along the direction (main-scanning direction) perpendicular to the direction (sub-scanning direction) of transmitting the recording medium for ejecting ink droplets from a plurality of ejection orifices. During the non-recording period, on the other hand, the recording medium is shifted at a distance corresponding to the recording width of the recording medium. Therefore, the ink jet recording method is responsible for ejecting ink droplets on the recording medium in accordance with the recording signals and has several advantages over the others in that it provides more excellent image qualities and performs the high-speed printing with a low-running cost. By using the recording head comprising a plurality of ink-ejecting nozzles being arranged in the sub-scanning direction, an image having a width corresponding to the number of the nozzles can be recorded by one scanning movement of the recording head in accordance with the ink jet method, resulting in that the high-speed printing is attained.
In the case of the ink jet recording apparatus for performing a color printing, furthermore, a colored image can be obtained by using droplets to be ejected from different recording heads corresponding to different colors, respectively, and placing one droplet upon another. In general, a recording apparatus capable of forming a full color image comprises recording heads and ink tanks which are corresponding to the number of color inks to be required. For example, the number of recording heads is three or four when three primary colors, i.e., yellow (Y), magenta (M), and red (R), or with black (B) are used, respectively. In resent years, the above kind of recording apparatus capable of forming a full color image with a recording head unit adapted to eject three or four kinds of ink has been put in practical use.
Besides, the ink jet recording apparatus easily records an image on a larger sized recording medium such as a sheet of A1 sized paper or the like. That is, a recording system for recording A1 sized color images, connecting with a device for reading an image to reproduce an original, such as a plotter (i.e., a printer used as an output device of the CAD system or the like), has been commercially available. Furthermore, there is a growing demand of applying the ink jet recording method in various fields. In the case of a presentation, for example, the ink jet recording method is applied in the process of recording an image on an OHP film to be projected for the presentation on a conference, a lecture, or the like. To meet the above demands, the ink jet recording apparatus has been developed and commercially manufactured to perform an excellent image formation without depending on a kind of the recording medium in spite of the fact that it is selected from the media having different properties of absorbing ink.
Accordingly, there is a sharp rise in demand of the ink jet recording method not only in the field of data-processing but also extensively in the field of using ink-supporting medium (e.g., a sheet of paper, an OHP sheet, or a sheet of cloth) to be given ink or the like, such as textile and clothing industries, as an outstanding recording means. In addition, an image with excellent qualities has been desired in every fields.
We are now explaining how to form a gradation (hereinafter, also referred as a gray level or a gray scale) of an image by the ink jet recording apparatus described above. For the gradation, several methods have been proposed, for example an area factor method in which the entire gray scale of an image is represented by one picture element of the plurality of picture elements consisting of only one of two levels of the scale in accordance with a predetermined threshold; a liquid droplet modification method (Japanese Patent Application Laid-open No. 59-207265 and Japanese Patent Application Laid-open No. 57-160654) in which the size of each droplet to be attached on the recording medium is changed by modifying the volume of liquid droplet; and a multi-droplet method in which a plurality of liquid droplets are successively ejected onto a same point to form one dot with a gradation thereof determined by the number of the droplets. Among these methods, however, the area factor method brings with a decline in the image-resolution, and also the liquid droplet modulation method cannot take a range of the gradation within the practical use, as for example, it is difficult to modify the volume of a liquid droplet. In the case of the multi-droplet method, on the other hand, it is possible to perform an image formation with a high-resolution and a high gradation by selectively using an ink jet recording head capable of ejecting smaller sized droplets compared with that of the other methods.
Next we are going to explain the image formation in accordance with the multi-droplet method, briefly. As already described above, each pixel of the image is formed by placing a plurality of droplets one on top of the other or on top of one another. In this case, a pixel formed by only one droplet is also used for the image formation.
After placing the liquid droplet on the recording medium, the liquid droplet requires over several hundred milliseconds of time to sink into the medium. As a result, a semicircular pattern of the permeation can be observed as a cross sectional view after placing the liquid droplet on the recording medium and thus a larger sized semicircular pattern is obtained by placing a subsequent droplet upon a prior droplet. Consequently, each size of the pixel can be individually determined by selecting the number of the droplets to be placed on the recording medium.
It is expected that the recording speed is decreased when the number of the droplets to be piled up is increased for obtaining a more extended extend range of the image's gradation. However, this problem can be solved by combining the several methods, such as the liquid droplet modification method described above and another method described in Japanese Patent Application Laid-open No. 63-502261 in which a plurality of droplets respectively having different volumes is ejected from nozzles of the ink jet head and piled on the recording medium within an extremely short time.
The recording process in accordance with the multi-droplet method will be explained in the following description.
FIG. 1 is a perspective illustration for explaining the construction of a recording head and its peripheral structure in the conventional ink jet recording apparatus.
The recording head 1 is mounted on a carriage 4 slidably engaged on guide shafts 5A and 5B. These shafts 5A and 5B serve to guide a shuttle movement of the carriage 4 in the direction of an arrow A (i.e., a main-scanning direction) in FIG. 1 with the aid of a driving means (not shown). Consequently, the recording head 1 is able to record an input image data on a recording medium 2 by its reciprocating motion in company with the carriage 4.
The recording head 1 has a plurality of orifices for ejecting ink droplets. In this example, thirty-two orifices are formed and arranged in the direction of transporting the recording medium 2, defined as a sub-scanning direction B perpendicular to the above direction A. In the arrangement, furthermore, the orifices are in close formation with the density of 400 dpi (dot per inch) and the distance of 63.5 .mu.m between them.
Each orifice communicates with a corresponding ink path where a heater element is installed for generating thermal energy to be used for ejecting an ink droplet. That is, the heater element causes heat by receiving an electric pulse in accordance with the driving data. Then the heat causes a film-boiling phenomena in the ink to generate a bubble responsible for pushing a part of ink out from the orifice. In general, a driving frequency of the heater element (i.e., an ejection frequency) is 10 kHz. Furthermore, a size of each ink droplet can be determined so as to express a desired concentration with an optical density (OD) of 1.4 when three ink droplets are placed on the recording medium to form a pixel covering an area of 63.5 micrometers square. In this case, the image can be formed by four different types of pixels. That is, the number of droplets in one pixel takes a value selected from 0, 1, 2, and 3, corresponding to four levels of the gradation, respectively. Consequently, a substantial frequency for forming a pixel takes a value of 10/3 kHz as a result of using the recording head with the ejection frequency of 10 kHz for ejecting three ink droplets at the maximum to form one pixel.
As described above, the carriage 4 having the ink jet head 1 mounted thereon is guided by the guide shafts 5A and 5B to shift its position. The carriage's movement is performed with the aid of a timing belt having a part fixed with the carriage 4. The timing belt is being pulled by a pair of pulleys to be driven by a rotation of a motor (not shown).
An ink tube 6 is connected with the recording head 1 to supply ink from an ink tank (not shown), while a flexible cable 7 is connected with the recording head 1 having a head-driving circuit (a head driver) that receives a driving signal and a control signal according to recording data from a host system or an internal control unit through the cable. It is noted that these constituents 6 and 7 are made of flexible materials so as to follow in the movement of the carriage 4.
A platen roller 3 is in the type of a rolling cylinder having a longitudinal axis parallel to the guide shafts 5A and 5B. The platen roller 3 is driven by a sheet-feeder motor (not shown) to feed a sheet of recording paper 2 provided as the recording medium and is responsible for positioning and holding a recording surface thereof. Accompanying with the movement of the carriage, subsequently, the recording head 1 ejects ink droplets onto the recording surface facing the orifices.
FIG. 2 is a block diagram for explaining a control mechanism of the ink jet recording apparatus shown in FIG. 1.
A main controller 112 comprises a central processing unit (CPU) and the like to store image data (8 bit and 256 level gradation for each pixel) from a host computer 111 into a frame memory 113. With a predetermined timing, the main controller 112 transfers the image data corresponding to each pixel from the frame memory 113 to a drive data RAM 115 through a halftoning unit 114, resulting that the frame memory 113 stores the image data. In the halftoning unit 114, the image data corresponding to each pixel is converted into four-level (quaternary) data (2 bit for each pixel, 0-3 levels of the gray scale). The halftoning unit realizes an algorithm known as a area factor method or an error-diffusion method by means of an electric circuit. According to a control signal from the main controller 112, the driver controller 116 reads drive data stored in the drive data RAM 115 with respect to a series of numbers used for identifying orifices of the recording head 1. Then the read data is supplied into a head driver 117 to control its drive timing.
According to the above construction, the main controller 112 controls the events of: an ink ejection by the recording head 1 through the driver controller 116; a revolution of the carriage motor 121 through the carriage motor driver 119; and a revolution of the sheet feeding motor 122 through 122 the sheet feeding driver 120, to successively record characters, pictures, drawings, or the like on the recording paper 2 in according with the image data.
By the way, the recording apparatus is driven by one of several printing modes. However, this kind of printing control is not limited in the multi-droplet method but also other methods. A high speed printing mode that is called a draft mode (or an ink saving mode) has been used when the user verifies the printed image or performs other purpose. That is, the draft mode may be selected when the user requires a printing result as soon as possible by performing a higher speed printing compared with that of the normal mode, when the user requires only a printing result as an unimportant output, or when the user requires saving the ink.
FIG. 3 is a schematic illustration for explaining an image recording under the draft mode in accordance with a conventional ink jet recording method. In the figure, reference numerals from #1 to #32 indicate orifices of the recording head 1, respectively, while reference numeral 500 indicates an area of the recording medium. The recording head 1 moves in the direction of an arrow A'. As shown in the figure, the draft mode is responsible for thinning down the overwhelming number of pixels in the image to carry out the high speed printing. In the figure, each pixel is not formed by two or more ink droplets (i.e., it is not depended on the multi-droplet method) but is formed by one droplet at the maximum. That is, each pixel is printed as a result of binarized image data, so that it is formed by ejecting one droplet or not.
The following description is for explaining why the printing speed is improved by thinning down the overwhelming number of pixels in the binarized image.
In FIG. 3, the image 500 consists of a plurality of rows (L1-L32) each made up of a series of pixels placed next to each other so as to form a checkered pattern. In the figure, an open square represents a pixel where an ink droplet cannot be placed; and a crosshatch square represents a pixel where an ink droplet can be placed when the image data is provided.
During the recording procedure, the recording head ejects an ink droplet onto a checkered pattern's point (i.e., a pixel indicated by the crosshatch square in the figure) on the recording medium when the drive data is provided. However, the recording head does not eject any ink droplet onto adjacent checkered pattern's point (i.e., a pixel indicated by the open square in the figure) on the recording medium in spite of receiving the drive data. In the case of an orifice #1 of the recording head 1, for example, an ink droplet can be ejected onto a pixel #501 when the drive data is provided while it cannot be ejected when the drive data is not provided. On the other hand, a pixel #502 adjacent to the pixel #502 cannot receive any ink droplet regardless of receiving the drive data or not. Just as in the case of the pixels #501 and #502, furthermore, an ink droplet can be ejected onto a pixel #503 when the drive data is provided while it cannot be ejected when the drive data is not provided. On the other hand, a pixel #504 adjacent to the pixel #503 cannot receive any ink droplet regardless of receiving the drive data or not. Consequently, the orifice #501 ejects ink droplets on alternate pixels in the row L1 to form a series of pixels as a checkered pattern, regardless of receiving the drive data or not. Also, the other orifices performs the same recording as that of the orifice #501, so that they eject ink droplets on alternate pixels in their row to each form a series of pixels as a checkered pattern, regardless of receiving their drive data or not. By the way, the recording head has a heater's drive frequency (i.e., an ejection frequency) of 10 kHz. Therefore, each orifice may be 10 kHz in ejection frequency for two pixels when the draft mode corresponding to a binary image is performed. That is, the recording head 1 performs a recording movement two times faster as that of the standard, so that a speed of printing is improved.
However, if the pixels are thinned out to form a checkered pattern as shown in FIG. 3, pixels to be recorded are half of all of the pixels, so that it is difficult to make out characters or the like in the obtained image. As a matter of fact, the obtained image can be only used for roughly observing a layout of the paper and thus it comes to nothing or wastes the time of printing.
In the case of the multi-droplet method, in addition, a substantial increase of the printing speed cannot be attained because the recording head ejects a plurality of ink droplets on each pixel in spite of thinning out pixels to form a checkered pattern as shown in FIG. 3. Therefore, it is difficult to raise the ejection frequency of each nozzle, so that the printing speed cannot be increased.